U.S. patent number 5,438,998 [Application Number 08/117,869] was granted by the patent office on 1995-08-08 for broadband phased array transducer design with frequency controlled two dimension capability and methods for manufacture thereof.
This patent grant is currently assigned to Acuson Corporation. Invention is credited to Amin M. Hanafy.
United States Patent |
5,438,998 |
Hanafy |
August 8, 1995 |
**Please see images for:
( Certificate of Correction ) ** |
Broadband phased array transducer design with frequency controlled
two dimension capability and methods for manufacture thereof
Abstract
There is provided a transducer array with a plurality of
piezoelectric elements having a minimum and maximum thickness. In
one embodiment, the maximum thickness is less than or equal to 140
percent of the minimum thickness. In an alternate embodiment, the
maximum thickness is greater than 140 percent of the minimum
thickness and the transducer array is capable of simulating the
excitation of a wider aperture two-dimensional transducer array.
One or more matching layers may be used to further increase
bandwidth performance. In addition, a two crystal transducer
element as well as a composite transducer structure may be formed
using the principles of this invention.
Inventors: |
Hanafy; Amin M. (Los Altos
Hills, CA) |
Assignee: |
Acuson Corporation (Mountain
View, CA)
|
Family
ID: |
22375271 |
Appl.
No.: |
08/117,869 |
Filed: |
September 7, 1993 |
Current U.S.
Class: |
600/459;
310/334 |
Current CPC
Class: |
B06B
1/0622 (20130101); B06B 1/0644 (20130101); G10K
11/32 (20130101); H04R 17/08 (20130101) |
Current International
Class: |
B06B
1/06 (20060101); G10K 11/32 (20060101); G10K
11/00 (20060101); H04R 17/04 (20060101); H04R
17/08 (20060101); A61B 008/00 () |
Field of
Search: |
;128/661.01,662.03
;73/633 ;310/334,368,369 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Schrope, Simulated Capillary Blood Flow Measurement Using A
Nonlinear Contrast Agent, 1992, pp. 134-158. .
Newnham, Connectivity and Piezoelectric-Pyroelectric Composites,
1978, pp. 525-536. .
Swartz, R. et al., "Generation . . . with PVF.sub.2 ", IEEE Trans.
vol. SU-27 #6, Nov. 1980, pp. 295-303..
|
Primary Examiner: Jaworski; Francis
Attorney, Agent or Firm: Willian Brinks Hofer Gilson &
Lione
Claims
I claim:
1. A transducer for producing an ultrasound beam upon excitation
comprising:
a plurality of piezoelectric elements, each of said elements
comprising a thickness at at least a first point on a surface
facing a region of examination being less than a thickness at at
least a second point on said surface, said surface being generally
non-planar, said surface having a radius of curvature along an
elevation direction which is different than a radius of curvature
along an azimuthal direction.
2. The transducer of claim 1 wherein the surface of said each of
said elements acts to produce an exiting pressure wave comprising
at least two peaks.
3. The transducer of claim 1 wherein said surface is a curved
surface.
4. The transducer of claim 3 further comprising a back portion
opposing said surface, said back portion being a generally planar
surface.
5. The transducer of claim 3 further comprising a back portion
opposing said surface, said back portion being concave in
shape.
6. The transducer of claim 3 further comprising a back portion
opposing said surface, said back portion being convex in shape.
7. The transducer of claim 3 further comprising an acoustic
matching layer positioned between a body being examined and at
least one of said elements.
8. The transducer of claim 7 wherein said matching layer has a
matching layer thickness LML approximated by (1/2)(LE)(CML/CE),
where, for a given point on the transducer surface, LML is the
thickness of the matching layer, LE is the thickness of the
transducer element, CML is the speed of sound of the matching
layer, and CE is the speed of sound of the element.
9. The transducer of claim 8 further comprising a coupling element
disposed on said matching layer comprising acoustic properties
similar to said body being examined.
10. The transducer of claim 9 wherein a surface of said coupling
element is slightly concave in shape.
11. The transducer of claim 3 wherein said curved surface of said
element enables said element to be operable at a dominant
fundamental harmonic frequency and is operable at a dominant second
harmonic frequency.
12. The transducer of claim 1 wherein each of said elements is
plano-concave.
13. The transducer of claim 12 wherein each of said elements
further comprises side portions at each end of said element, said
thickness being a maximum near said side portions of each of said
elements and said thickness being a minimum substantially near a
center of each of said elements.
14. The transducer of claim 13 wherein said element is formed of
one of lead zirconate titanate, composite material, and
polyvinylidene fluoride.
15. An ultrasound transducer comprising:
a plurality of piezoelectric elements each comprising a front
portion facing a region of examination, a back portion, two side
portions, and a thickness between said front portion and said back
portion;
said thickness being greater at each of said side portions than
between said side portions;
said front portion being generally non-planar, said front portion
having a radius of curvature along an elevation direction which is
different than a radius of curvature along an azimuthal
direction;
wherein each of said elements produces an ultrasound beam having a
width which varies inversely as to a frequency of excitation of a
given element.
16. The transducer of claim 15 wherein each of said elements is
plano-concave.
17. The transducer of claim 16 further comprising at least one
acoustic matching layer positioned between a body being examined
and at least one of said elements.
18. The transducer of claim 15 wherein each of said curved surface
of said elements enables said element to be operable at a dominant
fundamental harmonic frequency and is operable at a dominant second
harmonic frequency.
19. A transducer for producing an ultrasound beam upon excitation
at a given frequency comprising:
a piezoelectric element comprising a front portion facing a region
of examination being generally non-planar, said front portion
having a radius of curvature along an elevation direction which is
different than a radius of curvature along an azimuthal direction,
wherein said element operates at a dominant fundamental harmonic
frequency and a dominant second harmonic frequency.
20. The transducer of claim 19 wherein said element is
plano-concave.
21. An ultrasound transducer comprising:
a plano-concave piezoelectric element comprising a curved front
surface facing a region of examination, a back surface, two sides,
and a thickness between said front surface and said back surface,
said front surface comprising a radius of curvature approximated by
the equation h/2+(w.sup.2 /8h), where h is the difference between a
minimum and maximum thickness of said transducer element and w is
the width of said transducer element between said sides, wherein
said element produces an ultrasound beam having a width which
varies inversely as to a frequency of excitation of said
element.
22. The transducer of claim 21 wherein said curved surface of said
element enables said element to be operable at a dominant
fundamental harmonic frequency and is operable at a dominant second
harmonic frequency.
23. An array-type ultrasonic transducer comprising:
a plurality of transducer elements disposed adjacent to one
another, each of said elements comprising a front portion facing a
region of examination, a back portion, two side portions, and a
transducer thickness between said front portion and said back
portion,
said transducer thickness being a maximum thickness at said side
portions and a minimum thickness between said side portions, said
maximum thickness being less than or equal to 140% of said minimum
thickness.
24. The transducer of claim 23 wherein said maximum thickness is
less than or equal to 140% of said minimum thickness and greater
than or equal to 120% of said minimum thickness.
25. The transducer of claim 23 further comprising a curved acoustic
matching layer disposed on said front portion of each of said
elements, said matching layer comprising a matching layer thickness
LML approximated by (1/2)(LE)(CML/CE), where, for a given point on
the transducer surface, LML is the thickness of the matching layer,
LE is the thickness of the transducer element, CML is the speed of
sound of the matching layer, and CE is the speed of sound of the
element.
26. The transducer of claim 23 wherein said elements are comprised
of PZT and are plano-concave in shape, said front portion being
curved in surface, and said minimum thickness being substantially
near a center of each of said elements.
27. An ultrasound system for generating an image comprising:
transmit circuitry for transmitting electrical signals to a
transducer probe;
a transducer probe for transmitting an ultrasound beam produced by
a given frequency excitation and for receiving pressure waves
reflected from a body being examined;
receive circuitry for processing the signals received by said
transducer probe;
a display for providing an image of an object being observed;
said transducer probe comprising a plurality of piezoelectric
elements, each of said elements comprising a thickness at at least
a first point on a surface facing a region of examination being
less than a thickness at at least a second point on said surface,
said surface being generally non-planar and having a radius of
curvature along an elevation direction which is different than a
radius of curvature along an azimuthal direction, wherein said
ultrasound beam has a width which is related to said frequency of
excitation of said element.
28. The system of claim 27 wherein each of said elements is
plano-concave.
29. The system of claim 28 further comprising an acoustic matching
layer positioned between said body being examined and at least one
of said surfaces.
30. A method of making a transducer for producing an ultrasound
beam upon excitation comprising the steps of:
forming a plurality of piezoelectric elements, each of said
elements comprising a thickness at at least one point on a surface
facing a region of examination being less than a thickness at at
least one other point on said surface such that an aperture of said
ultrasound beam varies inversely as to a frequency of excitation of
each of said elements, said surface being generally non-planar and
having a radius of curvature along an elevation direction which is
different than a radius of curvature along an azimuthal direction;
and
establishing an electric field through at least one portion of each
of said elements.
31. The method of claim 30 wherein said step of establishing an
electric field comprises placing a first electrode on each of said
surfaces and placing a second electrode on a portion opposing each
of said surfaces.
32. The method of claim 31 further comprising the step of placing
an acoustic matching layer positioned between an object being
examined and at least one of said elements.
33. The method of claim 32 wherein said matching layer has a
matching layer thickness LML approximated by (1/2)(LE)(CML/CE),
where, for a given point on the transducer surface, LML is the
thickness of the matching layer, LE is the thickness of the
transducer element, CML is the speed of sound of the matching
layer, and CE is the speed of sound of the element.
34. The method of claim 33 further comprising the step of placing a
coupling element comprising acoustic properties similar to said
object being examined on said matching layer.
35. The method of claim 34 wherein a surface of said coupling
element is slightly concave in shape.
36. A method of making a transducer for producing an ultrasound
beam upon excitation comprising the steps of:
forming a plurality of transducer elements disposed adjacent to one
another, each of said elements comprising a front portion facing a
region of examination, a back portion, two side portions, and a
transducer thickness between said front portion and said back
portion, said transducer thickness being a maximum thickness at
said side portions and a minimum thickness between said side
portions, said maximum thickness being less than or equal to 140%
of said minimum thickness; and
establishing an electric field through at least one portion of each
of said elements.
37. The method of claim 36 further comprising the step of placing
an acoustic matching layer positioned between an object being
examined and at least one of said elements.
38. The method of claim 37 wherein said matching layer has a
matching layer thickness LML approximated by (1/2)(LE)(CML/CE),
where, for a given point on the transducer surface, LML is the
thickness of the matching layer, LE is the thickness of the
transducer element, CML is the speed of sound of the matching
layer, and CE is the speed of sound of the element.
39. A method of producing an image in response to excitation of a
transducer for generating an ultrasound beam comprising the steps
of:
providing electrical signals to a transducer probe for transmitting
a beam of ultrasound pressure waves to a body being examined such
that said transducer probe includes a plurality of piezoelectric
elements, each of said elements comprising a thickness at at least
one point on a surface facing a region of examination being less
than a thickness at at least one other point on said surface, said
surface being generally non-planar and having a radius of curvature
along an elevation direction which is different than a radius of
curvature along an azimuthal direction, and an aperture of an
ultrasound beam varying inversely as to a frequency of excitation
of said element;
receiving pressure waves reflected from said body and converting
said received pressure waves into received electrical signals;
processing said received electrical signals; and
displaying the object being observed.
40. The method of claim 39 further comprising the step of placing
an acoustic matching layer between said object being observed and
at least one of said piezoelectric elements.
41. The method of claim 40 further comprising the step of placing a
coupling element comprising acoustic properties similar to a body
being examined on said matching layer.
42. The method of claim 41 wherein a surface of said coupling
element is slightly concave in shape.
43. The method of claim 42 further comprising the step of applying
said probe to said object and placing ultrasound gel between said
probe and said object.
44. A transducer having bandwidth activation energy for producing
an ultrasound beam comprising:
a plurality of piezoelectric elements each comprising a front
portion facing a region of examination, a back portion, two side
portions, and a thickness between said front portion and said back
portion;
said thickness being a maximum value LMAX near each of said side
portions and a minimum value LMIN between said side portions;
said front portion being generally non-planar;
wherein an increase in said bandwidth activation energy is
approximated by the ratio LMAX/LMIN.
45. The transducer of claim 44 further comprising two acoustic
matching layers positioned between a body being examined and at
least one of said elements.
46. The transducer of claim 44 wherein said transducer suppresses
the generation of reflections at an interface of said transducer
and an object being examined.
47. The transducer of claim 44 wherein a signal produced by said
transducer is stronger between said side portions than at said side
portions.
48. A transducer for producing an ultrasound beam upon excitation
comprising:
a plurality of piezoelectric elements, each of said elements
comprising a thickness at a first point on a surface facing a
region of examination being less than a thickness at a second point
on said surface, said surface being generally non-planar, said
thickness at said second point being less than or equal to 140% of
said thickness at said first point;
wherein each of said elements produces an ultrasound beam having a
width which varies inversely as to a frequency of excitation of a
given element.
49. The transducer of claim 48 wherein said thickness at said
second point is less than or equal to 140% of said thickness at
said first point and greater than or equal to 120% of said
thickness at said first point.
50. The transducer of claim 48 further comprising a curved acoustic
matching layer disposed on said surface of each of said elements,
said matching layer comprising a matching layer thickness LML
approximated by (1/2)(LE)(CML/CE), where, for a given point on the
transducer surface, LML is the thickness of the matching layer, LE
is the thickness of the transducer element, CML is the speed of
sound of the matching layer, and CE is the speed of sound of the
element.
51. A transducer for producing an ultrasound beam upon excitation
comprising:
a plurality of piezoelectric elements each comprising a front
portion facing a region of examination, a back portion, two side
portions, a center portion between said side portions, and a
thickness between said front portion and said back portion, said
thickness being greater at each of said side portions than between
said side portions, said front portion being generally non-planar
and having a radius of curvature along an elevation direction which
is different than a radius of curvature along an azimuthal
direction;
a plurality of first electrodes, each one of said first electrodes
disposed on said back portion of a corresponding one of said
piezoelectric elements;
a plurality of second electrodes, each one of said second
electrodes disposed between a body being examined and said front
portion of a corresponding one of said piezoelectric elements;
wherein an electric field between said first and second electrodes
is greater at said center portion than said side portions.
52. The transducer of claim 51 wherein the relationship of said
transducer suppresses portions to suppress the generation of
sidelobes.
53. The transducer of claim 51 wherein a signal produced by said
transducer is stronger between said side portions than at said side
portions.
54. The transducer of claim 51 wherein each of said elements is
plano-concave.
55. The transducer of claim 54 further comprising at least one
acoustic matching layer positioned between said body being examined
and at least one of said elements.
56. The transducer of claim 55 wherein said matching layer has a
matching layer thickness LML approximated by (1/2)(LE)(CML/CE),
where, for a given point on the transducer surface, LML is the
thickness of the matching layer, LE is the thickness of the
transducer element, CML is the speed of sound of the matching
layer, and CE is the speed of sound of the element.
57. The transducer of claim 51 wherein each of said elements
produces a beam having a narrow aperture at higher frequencies.
Description
Reference is made to copending application Ser. No. 08/117,868
filed Sep. 7, 1993 entitled Broadband Phased Array Transducer
Design with Frequency Controlled Two Dimension Capability and
Methods for Manufacture Thereof.
BACKGROUND OF THE INVENTION
This invention relates to transducers and more particularly to
broadband phased array transducers for use in the medical
diagnostic field.
Ultrasound machines are often used for observing organs in the
human body. Typically, these machines contain transducer arrays for
converting electrical signals into pressure waves. Generally, the
transducer array is in the form of a hand-held probe which may be
adjusted in position to direct the ultrasound beam to the region of
interest. Transducer arrays may have, for example, 128 transducer
elements for generating an ultrasound beam. An electrode is placed
at the front and bottom portion of the transducer elements for
individually exciting each element, generating pressure waves. The
pressure waves generated by the transducer elements are directed
toward the object to be observed, such as the heart of a patient
being examined. Each time the pressure wave confronts tissue having
different acoustic characteristics, a wave is reflected backward.
The array of transducers may then convert the reflected pressure
waves into corresponding electrical signals. An example of a
previous phased array acoustic imaging system is described in U.S.
Pat. No. 4,550,607 granted Nov. 5, 1985 to Maslak et al. and is
incorporated herein by reference. That patent illustrates circuitry
for combining the incoming signals received by the transducer array
to produce a focused image on the display screen.
Broadband transducers are transducers capable of operating at a
wide range of frequencies without a loss in sensitivity. As a
result of the increased bandwidth provided by broadband
transducers, the resolution along the range axis may improve,
resulting in better image quality.
One possible application for a broadband transducer is contrast
harmonic imaging. In contrast harmonic imaging, contrast agents,
such as micro-balloons of protein spheres, are safely injected into
the body to illustrate how much of a certain tissue, such as the
heart, is active. These micro-balloons are typically one to five
micrometers in diameter and, once injected into the body, may be
observed via ultrasound imaging to determine how well the tissue
being examined is operating. Contrast harmonic imaging is an
alternative to Thallium testing where radioactive material is
injected into the body and observed by computer generated
tomography. Thallium tests are undesirable because they employ
potentially harmful radioactive material and typically require at
least an hour to generate the computer image. This differs from
contrast harmonic imaging in that real-time ultrasound techniques
may be used in addition to the fact that safe micro-balloons are
employed.
In B. Schrope et al., "Simulated Capillary Blood Flow Measurement
Using a Nonlinear Ultrasonic Contrast Agent," Ultrasonic Imaging,
Vol. 14 at 134-58 (1992), which is incorporated herein by
reference, Schrope discloses that an observer may clearly see the
contrast agents at the second operating harmonic. That is, at the
fundamental harmonic, the heart and muscle tissue is clearly
visible via ultrasound techniques. However, at the second harmonic,
the observer is capable of clearly viewing the contrast agent
itself and thus may determine how well the respective tissue is
performing.
Because contrast harmonic imaging requires that the transducer be
capable of operating at a broad range of frequencies (i.e. at both
the fundamental and second harmonic), existing transducers
typically cannot function at such a broad range. For example, a
transducer having a center frequency of 5 Megahertz and having a
70% ratio of bandwidth to center frequency has a bandwidth of 3.25
Megahertz to 6.75 Megahertz. If the fundamental harmonic is 3.5
Megahertz, then the second harmonic is 7.0 Megahertz. Thus, a
transducer having a center frequency of 5 Megahertz would not be
able to adequately operate at both the fundamental and second
harmonic.
In addition to having a transducer which is capable of operating at
a broad range of frequencies, two-dimensional transducer arrays are
also desirable to increase the resolution of the images produced.
An example of a two-dimensional transducer array is illustrated in
U.S. Pat. No. 3,833,825 to Haan issued Sep. 3, 1974 and is
incorporate herein by reference. Two-dimensional arrays allow for
increased control of the excitation of ultrasound beams along the
elevation axis, which is otherwise absent from conventional
single-dimensional arrays. However, two-dimensional arrays are also
difficult to fabricate because they typically require that each
element be cut into several segments along the elevation axis,
connecting leads for exciting each of the respective segments. A
two-dimensional array having 128 elements in the azimuthal axis,
for example, would require at least 256 segments, two segments in
the elevation direction, as well as interconnecting leads for the
segments. In addition, they require rather complicated software in
order to excite each of the several segments at appropriate times
during the ultrasound scan because there would be at least double
the amount of segments which would have to be individually excited
as compared with a one-dimensional array.
Further, typical prior art transducers having parallel faces
relative to the object being examined tend to produce undesirable
reflections at the interface between the transducer and object
being examined, producing what is called a "ghost echo." These
undesirable reflections may result in a less clear image being
produced.
SUMMARY OF THE INVENTION
Consequently, it is a primary objective of this invention to
provide a broadband transducer array for use in an acoustic imaging
system that is easier and less expensive to manufacture.
It is also an objective of this invention to provide a broadband
transducer array capable for use in contrast harmonic imaging.
It is another objective of the present invention to provide a
transducer element and a matching layer both having a negative
curvature to allow for additive focusing in the field of
interest.
It is also an objective of the present invention to provide a
transducer array for use in an acoustic imaging system that is
capable of simulating a two-dimensional transducer array at least
at lower frequencies.
It is a further objective of the present invention to better
suppress the generation of undesirable reflections at the surface
of the object being examined.
It is another objective of the present invention to further
increase the sensitivity and bandwidth of the transducer by
disposing one or more matching layers on the front portion of a
piezoelectric layer that is facing a region of examination.
To achieve the above objectives, there are provided several
preferred embodiments of the present invention. In a first
embodiment of this invention, an array-type ultrasonic transducer
comprises a plurality of transducer elements disposed adjacent to
one another. Each of the elements comprises a front portion facing
a region of examination, a back portion, two side portions, and a
transducer thickness between the front and back portions. The
transducer thickness is a maximum thickness at the side portions
and a minimum thickness between the side portions. Further, the
maximum thickness is less than or equal to 140 percent of the
minimum thickness. Variation in thickness of the element along the
range axis as much as 20 to 40 percent is preferred in this
embodiment resulting in increased bandwidth and shorter pulse width
(i.e., the maximum thickness is between 120 and 140 percent the
value of the minimum thickness). This provides improved resolution
along the range axis.
In a second embodiment of this invention, a transducer for
producing an ultrasonic beam upon excitation comprises a plurality
of piezoelectric elements. Each of the elements comprises a
thickness at at least a first point on a surface facing a region of
examination being less than a thickness at at least a second point
on the surface, the surface being generally non-planar. In
addition, the aperture of an ultrasound beam produced by the
present invention varies inversely as to a frequency of excitation
of the element. Generally, where the maximum thickness of the
piezoelectric element is greater than 140 percent of the minimum
thickness of the piezoelectric element, the transducer may simulate
the beam produced by a two-dimensional array at lower frequencies.
This is due to the fact that at lower frequencies, the exiting
pressure wave generated by the transducer has at least two peaks.
Further, the full aperture is typically activated at lower
frequencies. Consequently, the second embodiment simulates the
excitation of a wider aperture two-dimensional transducer
array.
In a third preferred embodiment, a two crystal transducer element
design is provided comprising a first piezoelectric portion with a
thickness at at least one point on a first surface facing a region
of examination being less than a thickness at at least one other
point on the first surface, the first surface being generally
non-planar. An interconnect circuit may be disposed between the
first piezoelectric portion and a second piezoelectric portion. A
matching layer may be disposed on the first piezoelectric
portion.
In a fourth preferred embodiment, a composite structure transducer
is provided comprising a plurality of vertical posts of
piezoelectric material comprising varying thickness and polymer
layers in between the posts. This structure may be deformed to
produce the desired transducer configuration. In addition, a
matching layer may be disposed on the composite transducer
structure to further increase performance.
The transducer of all embodiments allows for the transducer to
operate at a broader range of frequencies and allows for correct
apodization. Because the embodiments do not require matching the
back acoustic port of the element, they generally are easier to
fabricate than prior art devices.
A first preferred method of the invention for making a transducer
is disclosed by forming a plurality of transducer elements disposed
adjacent to one another. Each of the elements comprises a front
portion facing a region of examination, a back portion, two side
portions, and a transducer thickness between the front and back
portions. Further, the transducer thickness is a maximum thickness
at the side portions and a minimum thickness between the side
portions, the maximum thickness being less than or equal to 140
percent of the minimum thickness. An electric field is established
through at least one portion of each of the elements.
A second preferred method of the invention for making a transducer
is disclosed by forming a plurality of piezoelectric elements, each
of the elements comprising a thickness at at least one point on a
front surface facing a region of examination being less than a
thickness at at least one other point on the surface, the surface
being generally non-planar. An electric field is established at
least through one portion of each of the elements. For example,
electrodes may be placed on the front surface and back portion of
each of the piezoelectric elements to provide the electric field.
Upon application of an excitation pulse to the electrodes, the
aperture of an ultrasound beam produced by the transducer varies
inversely as to the frequency of the excitation pulse, where the
maximum thickness of the piezoelectric element is typically greater
than 140 percent of the minimum thickness of the piezoelectric
element.
A third preferred method of the invention for making a transducer
is disclosed by forming a piezoelectric element comprising
composite material comprising a front portion facing a region of
examination, the thickness of at least one point on the front
portion being less than the thickness on at least one other point
on the front portion. First and second electrodes may also be
placed on the piezoelectric element. The element may be deformed to
the desired shape.
The transducer of all embodiments as well as those made by the
disclosed methods may be in the form of a hand-held probe which may
be adjusted in position during excitation to direct the ultrasound
beam to the region of interest. Further, the transducer of all
embodiments as well as those made by the disclosed methods may be
placed in a housing for placement in a hand-held probe. Other types
of probes and manners of directing the beam are possible. The
ultrasound system for generating an image comprises transmit
circuitry for transmitting electrical signals to the transducer
probe, receive circuitry for processing the signals received by the
transducer probe, and a display for providing the image of the
object being observed. The transducers convert the electrical
signals provided by the transmit circuitry to pressure waves and
convert the pressure waves reflected from the object being observed
into corresponding electrical signals which are then processed in
the receive circuitry and ultimately displayed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an ultrasound system for generating
an image.
FIG. 2 is a cross-sectional view of a transducer element in
accordance with the first preferred embodiment.
FIG. 3 is a cross-sectional view of a transducer element in
accordance with the second preferred embodiment.
FIG. 4 is a perspective view of a broadband transducer array
further illustrating the probe of FIG. 1 in accordance with the
first preferred embodiment.
FIG. 5 is a perspective view of a broadband transducer array
further illustrating the probe of FIG. 1 and the beam widths
produced for low and high frequencies in accordance with the second
preferred embodiment.
FIG. 6 is an enlarged view of a single broadband transducer element
of the transducer array constructed in accordance with the present
invention.
FIG. 7 is a perspective view of a broadband transducer array in
accordance with the present invention further illustrating the
probe of FIG. 1 and having a curved matching layer disposed on a
front portion of the transducer elements.
FIG. 8 is a cross-sectional view of a single broadband transducer
element in accordance with the present invention having a curved
matching layer and further having a coupling element thereon.
FIG. 9 is a view of the exiting beam width produced by the
broadband transducer elements from low to high frequencies as
compared to the width of the transducer element in accordance with
the second preferred embodiment.
FIG. 10 is an example of a typical acoustic impedance frequency
response plot resulting from operation of the transducer
constructed in accordance with the second preferred embodiment.
FIG. 11 is an example of a typical acoustic impedance frequency
response plot resulting from operation of a prior art
transducer.
FIG. 12 is a cross-sectional view of a two crystal design having
interconnect circuitry between the two crystal elements in
accordance with the third preferred embodiment.
FIG. 13 is a cross-sectional view of an alternate two crystal
design.
FIG. 14 is a cross-sectional view of a composite transducer element
in accordance with a fourth preferred embodiment.
FIG. 15 is a cross-sectional view of the composite transducer
element of FIG. 14 which is deformed.
FIG. 16 is a cross-sectional view of a piezoelectric layer and
surface grinder wheel illustrating a preferred method for machining
the surface of the piezoelectric layer.
FIG. 17 is a cross-sectional view of a piezoelectric layer and
surface grinder wheel illustrating another preferred method for
machining the surface of the piezoelectric layer.
FIG. 18 shows a partial perspective view of a linear transducer
array in accordance with the present invention.
FIG. 19 shows a partial perspective view of a curvilinear
transducer array in accordance with the present invention with a
portion of the flex circuit removed at one end for purposes of
illustration.
FIG. 20 shows an impulse response and the corresponding frequency
spectrum for the transducer element of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the accompanying drawing FIG. 1, there is provided
a schematic view of an ultrasound system 1 for generating an image
of an object or body 5 being observed. The ultrasound system 1 has
transmit circuitry 2 for transmitting electrical signals to the
transducer probe 4, receive circuitry 6 for processing the signals
received by the transducer probe, and a display 8 for providing the
image of the object 5 being observed.
Referring also to FIG. 4, the probe 4 contains an array 10 of
transducer elements 11. Typically, there are one hundred twenty
eight elements 11 in the y-azimuthal axis forming the broadband
transducer array 10. However, the array can consist of any number
of transducer elements 11 each arranged in any desired geometrical
configuration. The transducer array 10 is supported by backing
block 13.
The probe 4 may be hand-held and can be adjusted in position to
direct the ultrasound beam to the region of interest. The
transducer elements 11 convert the electrical signals provided by
the transmit circuitry 2 to pressure waves. The transducer elements
11 also convert the pressure waves reflected from the object 5
being observed into corresponding electrical signals which are then
processed in the receive circuitry 6 and ultimately displayed
8.
Referring to FIGS. 2, 4, and 6, there is provided the first
embodiment of the present invention. Transducer element 11 has a
front portion 12, a back portion 14, a center portion 19, and two
side portions 16 and 18. The front portion 12 is the surface which
is positioned toward the region of examination. The back portion 14
may be shaped as desired, but is generally a planar surface. The
front portion 12 is generally a non-planar surface, the thickness
along the z-axis of element 11 may be greater at each of the side
portions 16 and 18 and smaller between the side portions. In such a
configuration the radius of curvature along the elevation direction
is different from the radius of curvature along the azimuthal
direction. The term side portion 16, 18 refers not only to the
sides 15 of the respective element 11, but may also include a
region interior to the element 11 where the thickness of the
element is greater than a thickness toward the interior of the
element (e.g., where the thickness of each of the sides of the
element are tapered).
Although the front portion 12 is illustrated having a continuously
curved surface, front portion 12 may include a stepped
configuration, a series of linear segments, or any other
configuration wherein the thickness of element 11 is greater at
each of the side portions 16 and 18 and decreases in thickness at
the center portion 19, resulting in a negatively "curved" front
portion 12. The back portion 14 which is generally preferably a
planar surface may also be, for example, a concave or convex
surface.
Element 11 has a maximum thickness LMAX and a minimum or smallest
thickness LMIN, measured along the range axis. Preferably the side
portions 16 and 18 both are equal to the thickness LMAX and the
center of element 11, or substantially near the center of element
11, is at the thickness of LMIN. However, each of the side portions
16, 18 do not have to be the same thickness and LMIN does not have
to be in the exact center of the transducer element to practice the
invention.
In the first preferred embodiment, the value of LMAX is less than
or equal to 140 percent the value of LMIN. This allows for an
increase in bandwidth activation energy generally without the need
to reprogram the ultrasound machine for generating the ultrasound
beam. Further, when the value of LMAX is less than or equal to 140
percent the value of LMIN, the exiting beam width is generally the
same for different exciting frequencies.
The increase in bandwidth activation energy for the transducer
configuration of the present invention is approximated by LMAX/LMIN
where the transducer is of the free resonator type (i.e., does not
comprise a matching layer) or is an optimally matched transducer
(i.e., has at least two matching layers), to be discussed later. In
the first preferred embodiment shown in FIGS. 2, 4, and 6, the
bandwidth may be increased by 40 percent by increasing the
thickness of LMAX relative to LMIN by 40 percent, respectively
(e.g., LMAX is 140 percent of the value of LMIN).
If, for example, a transducer has an LMAX of 0.3048 mm and an LMIN
of 0.254 mm, the bandwidth is increased by 20 percent as compared
to a transducer having a uniform thickness of 0.254 mm. Similarly,
if a transducer has an LMAX of 0.3556 mm and an LMIN of 0.254 mm,
the bandwidth is increased by 40 percent as compared to a
transducer having a uniform thickness of 0.254 mm. Variation in
thickness of the element along the range axis as much as 20 to 40
percent is preferred in this embodiment resulting in increased
bandwidth and shorter pulse width (i.e., the maximum thickness is
greater than or equal to 120 percent of the minimum thickness or
less than or equal to 140 percent of the minimum thickness). This
results in the maximum bandwidth increase, approximately 20 to 40
percent, respectively. Further, this provides improved resolution
along the range axis.
The slight variation in thickness of the front portion 12 relative
to the back portion 14 of the first embodiment allows for better
transducer performance where, for example, the transducer is
activated at three different frequencies, such a 2 MHz, 2.5 MHz,
and 3 MHz, known as a tri-frequency mode of operation. Such a
tri-frequency mode of operation may be used in cardiac
applications. Moreover, the slight variation in transducer
thickness may also improve transducer performance for other
tri-frequency modes of operation, such as operation at the
frequencies of 2.5 MHz, 3.5 MHz, and 5 MHz.
Preferably, the element 11 is a plano-concave structure and is
composed of the piezoelectric material lead zirconate titanate
(PZT). However, the element 11 may also be formed of composite
material as discussed later, polyvinylidene fluoride (PVDF), or
other suitable material. Referring also to FIG. 8, electrodes 23
and 25 may appropriately be placed on the front 12 and bottom 14
portions of the element 11 in order to excite the element to
produce the desired beam, as is well known in the art. Although
electrode 25 is shown to be disposed directly on the piezoelectric
element 11, it may alternatively be disposed on matching layer 24.
As a result, the matching layer 24 may be directly disposed on
piezoelectric element 11. The electrodes 23 and 25 establish an
electric field through the element 11 in order to produced the
desired ultrasound beam.
An example of the placement of electrodes in relation to the
piezoelectric material is illustrated in U.S. Pat. No. 4,611,141 to
Hamada et al. issued Sep. 9, 1986 and is incorporated herein by
reference. A first electrode 23 provides the signal for exciting
the respective transducer element and the second electrode may be
ground. Leads 17 may be utilized to excite each of the first
electrodes 23 on the respective transducer elements 11 and the
second electrodes 25 may all be connected to an electrical ground.
As is commonly known in the industry, electrodes may be disposed on
the piezoelectric layer by use of sputtering techniques.
Alternatively, an interconnect circuit, described later, may be
used to provide the electrical excitation of the respective
transducer elements.
Referring now to FIGS. 3 and 5, there is shown the second preferred
embodiment of the present invention wherein like components have
been labeled similarly. Although FIGS. 6 and 8 have been described
in relation to the first preferred embodiment, they will be used to
illustrate the second preferred embodiment in light of the
similarity of the two embodiments. Further, the thickness at at
least a first point on the front portion 12 is less than a
thickness at at least a second point on the front portion. In
addition, the front portion is generally non-planar.
In the second preferred embodiment, the value of LMAX is greater
than 140 percent the value of LMIN. Where the value of LMAX is
greater than 140 percent of the value of LMIN, the exiting beam
width produced typically varies with frequency. In addition, the
lower the frequency, the wider the exiting beam width.
FIG. 9 illustrates the typical variation in the exiting beam width
or aperture along the elevation direction produced by the broadband
transducer from low to high frequencies in accordance with the
second preferred embodiment. At high frequencies, such as 7
Megahertz, the beam has a narrow aperture. When the frequency is
lowered, the beam has a wider aperture. Further, at low enough
frequencies, such as 2 Megahertz, the beam is effectively generated
from the full aperture of the transducer element 11. As shown in
FIG. 9, the exiting pressure wave has two peaks, simulating the
excitation of a wide aperture two-dimensional transducer array at
lower frequencies.
FIGS. 5 further illustrates the beam width variation of the whole
transducer array as a function of frequency for the second
preferred embodiment. At high excitation frequencies, the exiting
beam width has a narrow aperture and is generated from the center
of elements 11. On the contrary, at low excitation frequencies, the
exiting beam width has a wider aperture and is generated from the
full aperture of elements 11.
By controlling the excitation frequency, the operator may control
which section of transducer element 11 generates the ultrasound
beam. That is, at higher excitation frequencies, the beam is
primarily generated from the center of the transducer element 11
and at lower excitation frequencies, the beam is primarily
generated from the full aperture of the transducer element 11.
Further, the greater the curvature of the front portion 12, the
more the element 11 simulates a wide aperture two-dimensional
transducer array.
In order to pursue the second preferred embodiment, that is,
increasing the bandwidth greater than 40 percent, it may be
necessary to reprogram the ultrasound machine for exciting the
transducer at such a broad range of frequencies. As seen by the
equation LMAX/LMIN, the greater the thickness variation, the
greater the bandwidth increase. Bandwidth increases of 300 percent,
or greater, for a given design may be achieved in accordance with
the principles of the invention. Thus, the thickness LMAX would be
approximately three times greater than the thickness LMIN. The
bandwidth of a single transducer element, for example, may range
from 2 Megahertz to 11 Megahertz, although even greater ranges may
be achieved in accordance with the principles of this invention.
Because the transducer array constructed in accordance with this
invention is capable of operating at such a broad range of
frequencies, contrast harmonic imaging may be achieved with a
single transducer array in accordance with this invention for
observing both the fundamental and second harmonic (i.e., the
transducer is operable at a dominant fundamental harmonic frequency
and is operable at a dominant second harmonic frequency).
The thickness variation of the transducer element 11 greatly
increases the bandwidth, as illustrated in FIGS. 10 and 11. FIGS.
10 and 11 provide one example of the effect of utilizing a
plano-concave transducer element 11 on bandwidth performance and
results may vary depending on the particular configuration used.
FIG. 10 illustrates an impedance plot for a transducer element 11
produced in accordance with the second preferred embodiment of the
present invention having an outer edge thickness LMAX of 0.015
inches (0.381 mm) and a center thickness LMIN of 0.00428 inches
(0.109 mm). As can be seen, the element has a bandwidth from
approximately 3.5 Megahertz to 10.7 Megahertz. In contrast, a
conventional element having a uniform thickness of 0.381 mm
typically has a bandwidth of approximately 4.5 Megahertz to
approximately 6.6 Megahertz, as illustrated by FIG. 11. Thus, by
comparing .DELTA.f, which is the difference between f.sub.r, the
anti-resonant frequency (i.e., maximum impedance), and f.sub.r, the
resonant frequency (i.e., minimum impedance), a fractional
bandwidth of 100% is provided by the transducer element produced in
accordance with the present invention versus a fractional bandwidth
of approximately 38% for the prior art design.
Therefore, by controlling the curvature shape of the transducer
element (i.e., cylindrical, parabolic, gaussian, stepped, or even
triangular), one can effectively control the frequency content of
the radiated energy. The use of each of these shapes, as well as
others, is considered within the scope of the present
invention.
Referring now to FIGS. 7 and 8, wherein like components are labeled
similarly, the transducer structure in accordance with the
invention is shown having a curved matching layer 24 disposed on
the front portion 12 of transducer element 11. The matching layer
24 is preferably made of a filled polymer. Moreover, the thickness
of the matching layer 24 is preferably approximated by the
equation:
where, for a given point on the transducer surface, LML is the
thickness of the matching layer, LE is the thickness of the
transducer element, CML is the speed of sound of the matching
layer, and CE is the speed of sound of the element. The curvature
of the front portion 12 may be different than the curvature of the
top portion 26 of the matching layer 24 because the thickness of
the matching layer depends on the thickness of the element at a
given point of the transducer surface. Although one or more
matching layers are preferably formed using the above equation, the
matching layers may be constant in thickness for ease of
manufacturing.
By the addition of matching layer 24, the fractional bandwidth can
be improved. Further, the transducer may act with increased
sensitivity. However, the thickness difference between the edge and
center of the assembled substrates will control the desired
bandwidth increase, and the shape of the curvature will control the
base bandshape in the frequency domain. Further, because both the
transducer element 11 and the matching layer 24 have a negative
curvature, there is additive focusing in the field of interest.
More than one matching layer may be added to the front portion 12
to effect focusing in the field of interest and to improve the
sensitivity of the transducer. Preferably, there are two matching
layers placed upon the piezoelectric element 11 resulting in an
optimally matched transducer. Each are calculated by the equation
LML=(1/2)(LE)(CML/CE). Specifically, for calculating the thickness
LML for the first matching layer, the value of the speed of sound
CML for that first material is used. When calculating the thickness
LML for the second matching layer, the value of the speed of sound
CML for that second material is used. Preferably, the value of the
acoustic impedance for the first matching layer (i.e., the matching
layer closest to the piezoelectric element) is approximately 10
Mega Rayls and the value of the acoustic impedance for the second
matching layer (i.e., the matching layer closest to the object
being observed) is approximately 3 Mega Rayls.
A coupling element 27 having the acoustical properties of the
object being examined may be disposed on the matching layer or
directly on the second electrode 25 if, for example, the matching
layer is not used. The coupling element 27 may provide increased
patient comfort because it may alleviate any of the sharper
surfaces in the transducer structure which are in contact with the
body being examined. The coupling element 27 may be used, for
example, in applications where the curvature of the front portion
12 or top portion 26 are large. The coupling element 27 may be
formed of unfilled polyurethane. The coupling element may have a
surface 29 which is generally flat, slightly concave, or slightly
convex. Preferably, the curvature of surface 29 is slightly concave
so that it may hold an ultrasound gel 28, such as Aquasonic.RTM.
manufactured by Parker Labs of Orange, N.J., now shown, between the
probe 4 and the object being examined. This provides strong
acoustical contact between the probe 4 and the object being
examined. The matching layer and coupling element described may be
placed on all of the embodiments disclosed.
Machines such as a numerically controlled machine tool which is
commonly used in the ultrasound industry may be used to provide the
thickness variation of the transducer element. The machine tool may
machine an initial piezoelectric layer in order to have the desired
thickness variation of LMAX and LMIN.
FIG. 16 shows a first method of machining the piezoelectric layer
80 where it is desired to have a curvature 82 on the front portion.
The numerically controlled machine is first inputted with the
coordinates for defining the radius of curvature R approximated by
the equation h/2+(w.sup.2 /8h), where h is the thickness difference
between LMAX and LMIN and w is the width of the transducer element
along the elevation axis. Then, a surface grinder wheel 84 on the
numerically controlled machine having a width coextensive in size
with the piezoelectric layer 80 machines the piezoelectric layer.
The surface grinder wheel rotates about an axis 86 which is
parallel to the elevation axis. The surface grinder wheel contains
an abrasive material such as Aluminum Oxide. The surface grinder
wheel preferably begins machining at one end of the piezoelectric
layer 80 along the azimuthal direction until it reaches the other
end of the piezoelectric layer.
FIG. 17 shows an alternate method of machining the piezoelectric
layer 80. With this method, the surface grinder wheel 84 is tilted
such that one corner 88 of the surface grinder wheel contacts a
surface of the piezoelectric layer 80. For a given azimuthal
region, the surface grinder wheel 84 begins at one side of the
piezoelectric layer 80 along the elevation axis until it reaches
the other side of the piezoelectric layer along the elevation axis
(e.g., the surface grinder wheel makes the desired cut along the
elevation axis for a certain index in the azimuthal axis). The
surface grinder wheel 84 rotates about an axis 90. Then, the
surface grinder wheel 84 is moved to a different region or index
along the azimuthal axis and repeats the machining from one side to
the other side of the piezoelectric layer along the elevation axis.
This process is repeated until the whole piezoelectric layer 80 is
machined to have the desired curvature 82.
The machined surface may also be ground or polished to provide a
smooth surface. This is especially desirable where the transducer
is used at very high frequencies such as 20 MHz.
Referring also to FIGS. 7 and 18, a number of electrically
independent piezoelectric elements 11 may then be formed by dicing
kerfs 94 accomplished by dicing the piezoelectric material, as is
commonly done in the industry. The kerfs 94 result in a plurality
of matching layers 24, piezoelectric elements 11, and electrodes
23. The kerf may also slightly extend into the backing block 13 to
ensure electrical isolation between transducer elements.
Referring to FIG. 8, a metalization layer may be directly deposited
on top of the piezoelectric layer prior to dicing to form the
second electrodes 25. If a matching layer 24 is also employed, the
second electrode 25 is preferably disposed on the top portion 26 of
matching layer 24. However, the top portion 26 of the matching
layer 24 is preferably shorted to the second electrode 25 via
metalization across the edges of the matching layer or by using an
electrically conductive material such as magnesium or a conductive
epoxy. In addition, where a matching layer is used, the dicing may
be done after the matching layer is disposed on top of the
piezoelectric layer. In a preferred embodiment, the second
electrode 25 is held at ground potential. If a flex circuit 96,
described later, is used, the dicing may extend through the flex
circuit, forming individual electrodes 23.
When the transducer is designed for operation in the sector format,
the length S, which is the element spacing along the azimuthal
direction, is preferably approximated by half a wavelength of the
object being examined at the highest operating frequency of the
transducer. This approximation also applies for the two crystal
design described later. When the transducer is designed for linear
operation, or if the transducer array is curvilinear in form, the
value S may vary between one and two wavelengths of the object
being examined at the highest operating frequency of the
transducer.
FIG. 19 shows a curvilinear transducer array constructed in
accordance with the principles of this invention. Specifically, the
curvilinear array is constructed similarly to the linear transducer
array of FIG. 18. However, rather than directly resting on the
large backing block 13 of FIG. 18, the piezoelectric elements 11
and flex circuit 96 with corresponding electrodes 23 are placed
directly upon a first backing block 13' having a thickness of
approximately 1 mm. This allows easy bending of the array to the
desired amount in order to increase the field of view.
Typically, the radius of curvature of the first backing block 13'
is approximately 44 mm but may vary as desired. The first backing
block may be secured to a second backing block 13" having a
thickness in the range direction of approximately 2 cm by use of an
epoxy glue. Preferably, the surface of the second backing block 13"
adjacent to the first backing block 13' has a similar radius of
curvature. As is commonly know in the industry, a curvilinear array
functions similarly to a linear array having a mechanical lens
disposed in front of the linear array.
Because the signal at the center portion 19 of the transducer
element 11 is stronger than at the end or side portions 16 and 18,
correct apodization occurs (i.e, reduces or suppresses the
generation of sidelobes). This is due to the fact that the electric
field between the two electrodes on the front portion 12 and bottom
portion 14 is greatest at the center portion 19, reducing side lobe
generation. In addition, because the front and bottom portions are
not flat parallel surfaces, the generation of undesirable
reflections at the interface of the transducer and object being
examined (i.e., ghost echoes) are better suppressed. Further,
because the transducer array constructed in accordance with the
present invention is capable of operating at a broad range of
frequencies, the transducer is capable of receiving signals at
center frequencies other than the transmitted center frequency.
As to the design of the spacing between the elements 11 and the
design of the transducer aperture or width w, the upper operating
frequency of a transducer will have the greatest impact on the
grating lobe. The grating lobe image artifact (i.e., the creation
of undesirable multiple mirror images of the object being observed)
can be avoided if one designs the element spacing to take into
account the highest operating frequency for the transducer.
Specifically, the relationship between the grating lobe angle
.THETA..sub.g, the electronic steering angle in sector format
.THETA..sub.s, the wavelength of the object being examined at the
highest operating frequency of the transducer .lambda., and the
spacing between the elements S is given by the equation:
Therefore, for a given grating lobe angle, the design of the
transducer aperture is restricted by the upper operating frequency
of the transducer.
As illustrated by the equation, in order to sweep at higher
frequencies, it is necessary to reduce the aperture correlating to
that frequency. For example, at an operating frequency of 3.5
Megahertz, the desired spacing between the elements S is 220 um
while at 7.0 Megahertz, the spacing S is 110 um. Because at higher
frequencies it is desirable to decrease the aperture of the
transducer element as given by the above described equation, use of
the transducer element at lower frequencies will result in some
resolution loss. This is due to the fact that lower frequency
operation typically requires a greater element aperture. However,
this is compensated by the fact that the transducer simulates a
two-dimensional array at lower frequencies where the value of LMAX
is greater than 140 percent the value of LMIN, which increases the
resolution of the images produced at the lower frequencies by wider
aperture.
A two crystal transducer element design may be employed using the
principles of this invention. Referring to FIG. 12, a two crystal
transducer element 40 is shown having a first piezoelectric portion
42 and a second piezoelectric portion 44. These piezoelectric
portions may be machined as two separate pieces. Preferably, both
surfaces 46 and 48 are generated by the equation h/2+(w.sup.2 /8h),
where h is the thickness difference between LMAX and LMIN and w is
the width of the transducer element along the elevation axis.
Although piezoelectric portions 42 and 44 are illustrated as being
plano-concave in structure, the surfaces 46 and 48 may include a
stepped configuration, a series of linear segments, or any other
configuration. The thickness of each of the portions 42 and 44 may
be greater at each of the side portions 43, 45, 47, 49 and decrease
in thickness at the respective center portions of piezoelectric
portions 42 and 44. In addition, the back portions 51 and 53 of the
piezoelectric portions 42 and 44, respectively, are preferably
generally planar surfaces. However, these surfaces may also be
non-planar.
An interconnect circuit 50 is disposed between the first
piezoelectric portion 42 and the second piezoelectric portion 44.
The interconnect circuit 50 may comprise any interconnecting design
used in the acoustic or integrated circuit fields. The interconnect
circuit 50 is typically made of a copper layer carrying a lead for
exciting the transducer element 40. The copper layer may be bonded
to a piece of polyamide material, typically kapton. Preferably, the
copper layer is coextensive in size with each of the piezoelectric
portions 42 and 44. In addition, the interconnect circuit may be
gold plated to improve the contact performance. Such an
interconnect circuit may be a flex circuit manufactured by Sheldahl
of Northfield, Minn.
To further increase performance, a matching layer 52 may be
disposed above piezoelectric portion 42. Where both the first and
second piezoelectric portions are formed of the same material, the
matching layer 52 has a matching layer thickness LML approximated
by (1/2)(LE)(CML/CE), where, for a given point on the transducer
surface, LML is the thickness of the matching layer, LE is the
thickness of the first and second piezoelectric portions, CML is
the speed of sound of the matching layer, and CE is the speed of
sound of the piezoelectric portions. Ground layers 58 and 59 may be
disposed directly on the matching layer 52 and on surface 48,
connecting the two piezoelectric portions in parallel.
The matching layer may be coated with electrically conductive
material, such as nickel and gold. However, if the matching layer
52 is not employed, then the ground layers are both disposed
directly on the piezoelectric portions 42 and 44. The matching
layer 52 may face the region being examined. The transducer 40 may
be placed on a backing block 54, as is commonly used in the
ultrasonic field. Further, a coupling element as described earlier
may also be used.
FIG. 13 illustrates another two crystal design 55 employing the
principles of this invention. A first piezoelectric portion 56 and
a second piezoelectric portion 57 are provided. The piezoelectric
portion 56 is preferably plano-concave in shape. In addition, the
second piezoelectric portion 57 has a thickness variation along the
elevation direction as well. An interconnect circuit 50 as
described above may be used in between the two piezoelectric
portions to excite the two crystal transducer 55. A matching layer
as well as a coupling element as described earlier may also be
provided to improve performance as well as patient comfort.
Further, electrodes 58 and 59 may be used to connect the two
piezoelectric portions in parallel.
Preferably, the back portion 61 of the first piezoelectric portion
56 is generally a flat surface. The radius of curvature R for the
front portion 63 and the bottom portion 65 of the first and second
piezoelectric portions 56 and 57, respectively, is approximated by
the equation h/2+(w.sup.2 /8h), where h is the thickness difference
between LMAX and LMIN of piezoelectric portion 56 and w is the
width of the transducer element along the elevation axis.
Preferably, the value of LMAX and LMIN is the same for both the
first and second piezoelectric portions 56 and 57. The radius of
curvature R for the front portion 67 of the second piezoelectric
portion 57 is approximated by the equation h'/2+(w.sup.2 /8h'),
where h' is the thickness difference between the combined maximum
thickness for both piezoelectric portions and the combined minimum
thickness for both piezoelectric portions and w is the width of the
transducer element along the elevation axis. To achieve the desired
radii of curvature, piezoelectric portions 56 and 57 may be
machined by a numerically controlled machine tool as described
earlier.
Instead of using a uniform layer of piezoelectric material, a
composite structure 60 as shown in FIG. 14 may be utilized formed
of composite material. The composite structure 60 contains a
plurality of vertical posts or slabs of piezoelectric material 62
having varying thickness. In between the posts 62 are polymer
layers 64 which may be, for example, formed of epoxy material. The
composite material may, for example, be that described by R. E.
Newnham et al. "Connectivity and Piezoelectric-Pyroelectric
Composites", Materials Research Bulletin, Vol. 13 at 525-36 (1978)
and R. E. Newnham et al., "Flexible Composite Transducers",
Materials Research Bulletin, Vol. 13 at 599-607 (1978) which are
incorporated herein by reference. The composite structure 60 is
preferably plano-concave. An acoustic matching layer, not shown,
may be disposed on the front portion 66 for increasing
performance.
The composite material may be embedded in a polymer layer. Then,
the composite material may be ground, machined, or formed to the
desired size. In addition, the individual transducer elements may
be formed by sawing the composite structure, as is commonly done in
the ultrasound industry. The gaps between each of the respective
transducer elements may also be filled with polymer material to
ensure electrical isolation between elements.
Although the front portion 66 is shown as a curved surface, the
front portion 66 may include a stepped configuration, a series of
linear segments, or any other configuration wherein the thickness
of the structure 60 is greater at each of the side portions 70, 72
and decreases in thickness at the center. In addition, although the
back portion 68 is shown as a flat surface, the back portion may be
a generally planar surface, a concave or a convex surface.
Electrodes 74 and 76, similar to the electrodes described earlier,
may be placed on the front and back portions of the composite
structure.
The composite structure 60 of FIG. 14 may be deformed as shown in
FIG. 15 resulting in both a concave portion 66' and a concave
portion 68'. The deformed structure of FIG. 15 may result by
mechanically deforming the structure of FIG. 14. In certain
applications, the structure of FIG. 14 may be heated prior to
deforming. If the filler material between the vertical posts 62 is
made of silicone rather than an epoxy material, the structure of
FIG. 14 may easily be deformed without the application of heat. If
epoxy material is used, then the structure of FIG. 14 should be
exposed to approximately 50.degree. C. before deforming the
structure. In addition, the composite structure may be deformed in
the opposite direction, not shown, resulting in a concave portion
66' and a convex portion 68'. It should be noted that forming the
transducer structure of FIG. 14 not only allows for a broadband
transducer, but also generally provides focusing of the ultrasound
beam in the region of interest. By deforming the structure as shown
in FIG. 15, one is capable of "fine tuning" the focusing of the
ultrasound beam.
In operation, the transducer array 10 may first be activated at a
higher frequency along a given scan direction in order to focus the
ultrasound beam at a point in the near field. The transducer may be
gradually focused along a series of points along the scan line,
decreasing the excitation frequency as the beam is gradually
focused in the far field. Where the value of LMAX is greater than
140 percent the value of LMIN, the exiting beam width, which has a
narrow aperture at high frequencies, may widen in aperture as the
excitation frequency is decreased, as illustrated in FIG. 9.
Eventually, at a low enough frequency, such as two Megahertz, the
transducer 10 simulates a two-dimensional array by effectively
generating a beam using the full aperture of the transducer
elements 11. Further, the greater the curvature of front portion
12, the more the transducer 10 simulates a two-dimensional array. A
matching layer 24 may also be disposed on the front portion 12 of
element 11 in order to further increase bandwidth and sensitivity
performance.
In addition, when performing contrast harmonic imaging, the
transducer array elements 11 may first be excited at a dominant
fundamental harmonic frequency, such as 3.5 Megahertz, to observe
the heart or other tissue being observed. Then, the transducer
array elements 11 may be set to the receive mode at a dominant
second harmonic, such as 7.0 Megahertz, in order to make the
contrast agent more clearly visible relative to the tissue. This
will enable the observer to ascertain how well the tissue is
operating. When observing the fundamental harmonic, filters (e.g.,
electrical filters) centered around the fundamental frequency may
be used. When observing the second harmonic, filters centered
around the second harmonic frequency may be used. Although the
transducer array may be set to the receive mode at the second
harmonic as described above, the transducer array may be capable of
transmitting and receiving at the second harmonic frequency.
The application of pulses to obtain the desired excitation
frequency is well known in the art. For illustrative purposes,
referring now to FIG. 20, an impulse response 100 is shown having a
width of approximately 0.25 usec. The impulse response 100 is the
transducer response to an impulse excitation where LMIN is 0.109
mm, LMAX is 0.381 mm, and the radius of curvature of the front
portion 12 is 103.54 mm. The impulse response 100 results in a
frequency spectrum 102 ranging from approximately 1 MHz to 9 MHz.
It is desirable to excite the transducer element 11 with the use of
an impulse excitation when viewing the far field or in applications
where one is not limited to selecting a given aperture of the
transducer element 11 for producing an ultrasound beam. Exciting
the whole aperture of the transducer element 11 also helps produce
a finer resolution along the range axis.
To select the aperture of the central portion 19 of transducer
element when viewing the near field, a series of pulses,
approximately 2 to 5 pulses, may be used to excite the transducer
element 11. The pulses have a frequency correlating to the central
portion 19 of the element 11. Typically, the frequency of the
pulses is approximately 7 MHz and the width of the pulses is
approximately 0.14 usec.
To simulate a two-dimensional array at lower frequencies, as
discussed earlier, a series of pulses, approximately 2 to 5 pulses,
may be applied to excite the transducer element 11. The pulses have
a frequency which matches the resonance frequency correlating to
the thickest or side portions 16, 18 of the transducer element.
Typically, the frequency of the pulses is approximately 2.5 MHz and
the width of the pulses is approximately 0.40 usec. This helps
produce a clearer image when viewing the far field.
The elements 11 for the single crystal design shown in FIGS. 3, 5,
and 18 each measure 15 mm in the elevation direction and 0.0836 mm
in the azimuthal direction. The element spacing S is 0.109 mm and
the length of the kerf is 25.4 um. The thickness LMIN is 0.109 mm
and the thickness LMAX is 0.381mm. The radius of curvature of the
front portion 12 is 103.54 mm.
The backing block is formed of a filled epoxy comprising Dow
Corning's part number DER 332 treated with Dow Corning's curing
agent DEH 24 and has an Aluminum Oxide filler. The backing block
for a transducer array comprising 128 elements has dimensions of 20
mm in the azimuthal direction, 16 mm in the elevation direction,
and 20 mm in the range direction.
The shape and dimension of the matching layer 24 is approximated by
the equation LML=(1/2)(LE)(CML/CE) where, for a given point on the
transducer surface, LML is the thickness of the matching layer, LE
is the thickness of the transducer element, CML is the speed of
sound of the matching layer, and CE is the speed of sound of the
element. The transducers may be used with commercially available
units such as Acuson Corporation's 128 XP System having acoustic
response technology (ART) capability.
For the two crystal design of FIG. 12, the first and second
piezoelectric portions 42 and 44 have a minimum thickness of 0.127
mm and a maximum thickness of 0.2794 mm, as measured in the range
direction. The radius of curvature for the surfaces 46 and 48 of
piezoelectric portions 42 and 44 are 184.62 mm. The element spacing
S is 0.254 mm and the length of the kerf is 25.4 um.
For the two crystal design of FIG. 13, piezoelectric portions 56
and 57 have a minimum thickness of 0.127 mm and maximum thickness
of 0.2794 mm. The radius of curvature of the front portion 63 of
the first piezoelectric portion 56 and the back portion 65 of the
second piezoelectric portion is 184.62 mm. The radius of curvature
of the front portion 67 of piezoelectric portion 57 is 92.426
mm.
Finally, the composite structure design shown in FIG. 14 preferably
has dimensions similar to that for FIGS. 4 or 5, forming an array
of 128 transducer elements. The structure of FIG. 11 further
possesses a generally planar back portion 68 which is especially
desirable when focusing in the far field. The structure of FIG. 15
may be formed by deforming the ends of the structure of FIG. 14 in
the range direction. Where focusing in the near field at
approximately 2 cm into the body being examined, the side portions
of the structure of FIG. 14 should be deformed by approximately
0.25 mm relative to the center portion.
Each of the backing block, the flex circuit, the piezoelectric
layer, the matching layer, and the coupling element may be glued
together by use of any epoxy material. A Hysol.RTM. base material
number 2039 having a Hysol.RTM. curing agent number HD3561, which
is manufactured by Dexter Corp., Hysol Division of Industry,
Calif., may be used for gluing the various materials together.
Typically, the thickness of epoxy material is approximately 2
um.
The flex circuit thickness for forming the first electrode is
approximately 25 um for a flex circuit manufactured by Sheldahl for
providing the appropriate electrical excitation. The thickness of
the second electrode is typically 2000-3000 Angstroms and may be
disposed on the transducer structure by use of sputtering
techniques.
It should be noted that the transducer array constructed in
accordance with the present invention may be capable of operating
at the third harmonic, such as 10.5 Megahertz in this example. This
may further provide additional information to the observer.
Moreover, the addition of the matching layer 24 will enable the
transducer array to operate at an even broader range of
frequencies. Consequently, this may further enable a transducer of
the present invention to operate at both a certain dominant
fundamental and second harmonic frequency.
It is to be understood that the forms of the invention described
herewith are to be taken as preferred examples and that various
changes in the shape, size and arrangement of parts may be resorted
to, without departing from the spirit of the invention or scope of
the claims.
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